Transfer case or rear diff would be first guesses. Do you have a slip-yoke eliminator? If so, remove the rear driveshaft and take it for a spin in 4wd (in essence, front wheel drive), and also run thru the gears in 2wd (obviously not moving, since no rear driveshaft). That will partly help to determine if it's further up the driveline (t-case or tranny), or rear diff. If the sound is not present, it's *probably* the rear diff. I say probably, because some gremlins only come out to play when parts are under load, which the tranny and t-case would not be with the rear driveshaft disconnected(tranny problems will still surface in the scenario, where transfer case issues may not if it's specifically from the rear output).
How many miles are on the rig? Have you had any gearing changed or lockers/lsd installed recently?